In the historic sense, Xrays are the electromagnetic emanations produced when an electron that has kinetic energy of hundreds or thousands of electron volts is decelerated as it impinges upon dense materials or radiatively combines with a vacancy in an inner atomic shell. The wavelengths of such emanations normally lie within the range 0.001 nanometers (nm) and 50 nm, and, in the modern sense, Xrays (or X-radiation) can be considered to be any electromagnetic radiation, however produced, whose wavelength lies within this range. Over all but the last few ocataves of this range the index of refraction of most materials differs only very slightly from unity, and reflection and refractive phenomena can be observed only with special precaution. It is accordingly difficult to concentrate or focus the energy that may be contained in an Xray beam. However, over certain wavelength ranges Xrays can be focussed by passage though specially designed Fresnel zone plates or by reflection from lamelar mirrors analogous to the highly reflective dielectric multilayer mirrors used in lasers. The focussing action of a Fresnel zone plate results not from refraction, but from multiple-beam interference of the Xrays; that of the mirrors results also from multiple-beam interference, rather than directly via simple reflection. Because the features of a multiple-beam interference pattern are quite noticeably shifted by a change in the wavelength in which they are formed, the focal length of a zone plate depends strongly upon the wavelength of the radiation focussed; i.e., exhibits strong dispersion. The reflection efficiency of a lamellar mirror used to focus an oncoming Xray beam must also be strongly dependent upon either wavelength or angle of incidence; its curvature must therefore be limited as, consequently, will be either the sharpness of the bandwidth of its focus. Thus, either kind of focussing device may be of limited suitability when the X-radiation to be focussed is not monochromatic.
The smallness of the wavelengths of Xrays, which is at the root of the difficulty in refracting them, has also the consequence that Xrays can be easily collimated by passing them through a small opening or aperture in a screen of absorbing material. The diffraction of X-rays by passage through any hole of macroscopic size is inconspicuous, and the line defined by small, coaxial apertures in screens some distance apart will be closely followed by Xrays sent through them, which Xrays thereafter may be considered to form an Xray beam. If the last such aperture is made exceedingly small, however, the diffraction of Xrays can be observed as a spreading of the transmistted beam beyond the confines of that aperture's geometrical shadow. It is not commonly realized that diffraction from an aperture of suitable size can also act to concentrate or focus a beam of Xrays, and it appears not to have been recognized previously that the focal properties of an aperture can be made usefully less wavelength-sensitive than those of a Fresnel-zone plate. An aperture of appropriate design can then serve as a focussing lens for broad-band X-radiation.